An open-type permanent magnet direct-drive outer-rotor wind power generator in the conventional technology is cooled by natural air. An open-type structure is conducive to natural ventilation heat transfer, and facilitates the use of a permanent magnet material for magnetic poles to prevent magnetism reduction in case of an impermissible degree of temperature rising. However, the power generator is usually exposed to extremely severe environmental conditions (exposed to wind, frost, rain, snow, dust, salt fog, etc.).
Air-cooled generator insulation having a tendency to being moistened is decided by its operation status and structure. Since the generator insulation can only employ a solid insulation medium which is embedded in iron core slots, and cannot be immersed in an insulating oil like a transformer does, and also cannot be sealed in an airtight metal shell filled with SF6 gas like a full-closed gas insulated substation (GIS) does, but can only be exposed to the air. During normal operation, heat generated by an iron core and a winding of the generator needs to be brought away by flowing air. When heat generated by the generator and heat dissipated from the generator reach equilibrium, temperatures of the iron core and the winding of the generator are maintained within a certain numerical range. When the generator operates normally, the interior of the natural air cooled outer-rotor permanent magnet direct-drive generator also takes air from outdoors as a cooling medium. The temperatures of the iron core and the winding may be higher than the temperature of the air as the cooling medium. After the generator stops, the temperatures of the iron core and the winding drop gradually, and due to the effect of thermal expansion and contraction of the air inside the clearances and air gaps, a lot of air enters the generator to reach a pressure balance. In such a case, the insulation absorbs moisture in the air to be moistened, and if it is in a thunderstorm season, the air humidity may be larger after the rain, and the insulation of the generator will be moistened even more seriously. After the insulation of the generator is moistened, the leakage current is scores of times or even hundreds of times of the normal value, and the insulation resistance is a few tenths of the normal value. According to data analysis, if the generator insulation is seriously moistened, it cannot operate if not performed with a drying treatment. Generator insulation being moistened seriously is considered from the perspective of insulation test data. In fact, at the early stage of the insulation being moistened, only the surface of the insulation adsorbs moisture, and the interior of the insulation has not been moistened, the moisture on the surface of the insulation is tiny and is much easier to dry compared with the case of the insulation being immersed by water.
When the air has a large humidity, the reduction of the insulation resistance, resulted from the insulation of the generator being moistened, takes a short time, e.g. one day or even several hours. Thus it is required that the rain is restricted to enter the generator in rainy days or the wet air inside the generator is taken away timely after the rain.
For the open-type nacelle outer-rotor wind power generator, if a contact seal is employed, the interior of the generator cannot be directly cooled by relatively dry air flow for a long time in the dry time when it is not rainy or snowy.
At present, thermal power generation units, hydroelectric power generation units and nuclear power units operating in the power grid are usually arranged in a fixed plant. Generally, the plant may not suffer intrusion of rain and snow. Even if the hydroelectric power generation units are flooded, and the cooling medium (water) used by the above generator units is leaked, maintenance of the operating condition of the power generation units operating on the ground is far more convenient than that of onshore or offshore wind power generators operating in the wind plant. In the aspect of generator cooling, while convenience and superior performance of air-cooling in the natural environment can be taken advantage, the insulation level of the insulation system of the generator should be addressed and put to the test. The permanent magnet direct-drive outer-rotor wind power generator is exposed to wind, sand, rain, snow, sun exposure or freezing environment after downtime all the year round, which is drastically different from the environment where the ground turbo generators, gas turbine generators, hydraulic turbine generators are located, especially some repair works cost too much, and the crane use of the high-altitude operation (60 meters to 120 meters) requires a high payment. So the work which is easy to carry out on the ground becomes even impossible for the wind power generators. In another aspect, the operation in wind power generators is also dependent on windy weather. A wind turbine drives the generator rotor to rotate, and only after the generator stator senses the potential, a three-phase short circuit operation can be implemented at an exit of the stator, and the stator is dried by heat generated by the short-circuit current to improve the insulation level. Meanwhile, it also needs to implement pitch alternation based on the magnitude of the current wind speed to indirectly control the rotation speed of the generator, and further control the short circuit current and control the winding to generate heat to dry the moisture, and all of these conditions are dependent on the weather. Moreover, the duration of the wind affects the moisture drying effect, and the direct-drive outer rotor permanent magnet wind power generator has a large mass and requires an extremely large amount of the generated heat, and both the time for heat conduction after heat generation and the mass transfer drying time in moisture drying require several hours, thus the duration and intermittence of the wind both affect the moisture drying effect.
The inventors have found in the practical operation that the conventional technology has the following drawbacks.
(1) The permanent magnet direct-drive outer rotor wind power generator employs natural air to cool a stator iron core support and an outer wall of the rotor, and a certain amount of air in the natural environment intrudes into the cavity of the generator via the clearances between the stator and rotor of the wind power generator, and then flows to another end via the air gap in the axial direction to be gathered together, and light air after being gathered is forced out from a rear end sealed portion and is discharged into atmosphere. It is a gas (vapor)-liquid-solid multiphase fluid (including air, water vapor, rain, snow, salt fog, dust, floc, etc.) that flows through the internal clearances of the generator, which can cause deterioration of insulation performance, result in degradation of electrical properties and mechanical properties as well as reduction of residual pressure level and service life of the insulation of the generator, and eventually result in damage of insulation.
(2) The above description is involved in operations of ground power generation units. High-altitude operations with 60 to 120 meters, including the realization of various functions, especially the maintenance work carried out in the nacelle, usually cannot be implemented by human and material resources and even becomes impossible. Sealing, drying measures and maintenance (repair, replacement) of wind power generators are far more difficult than those of thermal power generators and hydroelectric power generators operating on the ground. Some of the good methods used in the ground power generation units are inconvenient to carry out and even difficult to apply to the wind power generation units operating in high altitude.
(3) The above-mentioned method of drying solely by hot air is only surface drying technology, and cannot meet the drying requirement after interlayer of the laminated sheets inside the stator iron core is moistened.
(4) The use of the open-type structure cannot withstand the hazards brought by air carrying rain or snow intruding into the generator in the storm weather or snowy weather, and the cooling of the generator is at the cost of reduction of the insulation level.
(5) After the shutdown, the humid air inside the cavity of the generator and the air gap is condensed to permeate into the generator, which may cause the coated layer on the surfaces of the generator stator and the permeate magnetic pole to be moistened, and may impact their service life.